Whenever load currents in excess of a few milliamps are required to drive a load, two design approaches are available: either a single driver element may be used that is adequate to provide the required power, or several smaller driver elements may be deployed in a parallel arrangement such that each provides a substantially equal share of the drive power. As is known in the art, even nominally identical active devices will exhibit slightly different characteristics, so a means of ensuring accurate load balancing is almost always required whenever overall efficiency of a parallel array of active drivers is of concern, otherwise each of the drivers must be over-designed to accommodate the worst-case load current imbalance that might arise.
Load balancing may be achieved either by ensuring that each driver device is very closely matched to every other, or by providing an adjustment means to calibrate the transfer function of each individual driver, or even some combination of both methods. Except in well-controlled monolithic implementations, it is often difficult to ensure, except perhaps through very careful test and selection procedures, that a group of nominally identical drivers do actually behave in a substantially identical manner over time and temperature. So as a practical matter, it is usually preferred to provide an auxiliary adjustment means: manual or “set-once” trims may be implemented. However, to ensure that the adjustment remains correct over time, feedback schemes are often preferred to dynamically establish the correct operating points for each driver element.
While driver elements that provide current-mode outputs may be connected in parallel without serious adverse effects, as shown for example in U.S. Pat. No. 6,344,774 B2 to Ghiozzi et al., it is generally not advisable to directly interconnect the outputs of voltage-mode driver elements. In the latter case, even small differences in the individual output voltages will give rise to undesirable cross-conduction currents between the drivers, and, if the impedance of the interconnection between any pair of voltage-mode drivers is too small, voltage differentials as small as a few millivolts can lead to catastrophic results. To mitigate this potential problem, ballast resistors are conventionally placed in series with the output of each voltage-mode driver element to limit the maximum cross-conduction currents. While this scheme will reduce the possibility of driver destruction, the efficiency of the design will be commensurately reduced since power will be wasted in the ballast resistors in direction proportion to the load current demands.
In general, small low power devices exhibit many superior performance characteristics in comparison to larger, higher power types. Therefore, when very wide bandwidth, low noise, low offset or low thermal drift performance is desired, it may be preferable to connect a plurality of small driver elements in parallel array to drive a common load. Despite that fact that more components are required for the parallel array implementation, it is often the case that the overall costs, physical space requirements and thermal dissipation problems may nevertheless be significantly reduced. In addition, a parallel architecture implicitly provides a degree of redundancy not present in a single-driver design.
Driver elements may be individual active devices such as bipolar transistors or FETs, monolithic devices such as amplifiers, or even entire active sub-circuits or modules.
When the drivers are discrete transistors, either bipolar or field-effect types, a modicum of load-balancing may be provided by simply placing a resistor in series with each emitter or source terminal, respectively. As is known in the art, current passing through this ballast resistor will develop a proportional voltage that serves as local negative feedback to the active device, mitigating both intrinsic and thermally related performance differences. In effect, the resistor serves to sense output current and provide feedback accordingly. When the drivers are active networks capable of both sourcing and sinking current, as for example linear amplifiers exhibiting very low output impedance, cross-conduction among the paralleled drivers may be likewise reduced by inserting a resistor in series with each driver.
When series ballast resistors are employed, their values should be selected to reduce current related errors to a safe level, and in general larger values provide better protection from current hogging and cross-conduction. Unfortunately, while the level of protection achieved increases as the value of these resistors is increased, the I2R power dissipated in these ballast resistors is wasted, so the designer is forced to trade off the degree of protection against the loss of efficiency.
In order to reduce the inefficiencies accompanying large-valued ballast resistors, several alternate schemes have been proposed, wherein the brute-force ballast resistor is replaced by or augmented with an active feedback network comprising both a sense resistor and at least one amplifying device that is configured to sense the voltage developed across the sense resistor by the load current and therefrom create a suitable feedback signal that is coupled to the driver to regulate its output.
When the driver element is a single bipolar transistor or an enhancement mode FET transistor (or an equivalent compound structure such as a Darlington), severe overload conditions may be prevented by placing a sensing resistor in series with the load current connected in a suitable configuration to a limiting transistor so that it turns on at a preset threshold and reduces the drive signal to the driver. While this method serves to protect drivers from severe overloading, it does not satisfactorily address the problems of cross-conduction or current-hogging when a plurality of drivers are operating in parallel to drive a common load. To mitigate these two problems, the feedback scheme must be capable of controlling and equalizing the outputs of the drivers continuously throughout the entire range of operation from quiescent conditions to full load operation.
An example of an active feedback scheme that addresses load sharing is disclosed in U.S. Pat. No. 4,035,715 to Wyman et al. for a power controller comprising a plurality of nominally similar driver modules. Two output current control schemes are presented: one teaches how the current of one module is sensed and used to control the total output of all the modules. By another, the output of each module is sensed individually and a composite feedback signal is developed to control the total load current. In this design, however, load-balancing among the drivers is only approximate, within ±10%, and is afforded by the known method of connecting series ballast resistors to the emitter terminal of each output transistor in each driver module, to provide local current feedback. Likewise, cross-conduction is limited only by the current-sensing series resistors themselves.
By employing high gain operational amplifiers to provide local negative feedback around a discrete output driver such as a bi-polar transistor or FET, accurate control may be achieved over the full operating range of the driver. An example of this method is provided in U.S. Pat. No. 6,646,508 to Barbetta, wherein a plurality of enhancement mode MOSFETs are disposed with their drain terminals connected together to serve as a push-pull output stage of a voltage-mode amplifier for audio applications. Each MOSFET is equipped with a source resistor coupled with an operational amplifier configured to both detect the voltage drop across the resistor as well as to provide gate drive to the MOSFET so that it behaves as a voltage controlled current source that output a current under the control of a separate driving signal. Because of the precise control effected for each individual driver, this method does prevent current-hogging among the driver elements, but it does not specifically address the issue of cross-conduction among device of like polarity.
When minimal cross-conduction errors and high quality dynamic performance are required but current-mode drivers are not a practical option, it is often preferable to employ a plurality of smaller, high quality voltage-mode drivers or amplifiers disposed in an efficient parallel configuration. Therefore, what is needed in the art is an efficient means to substantially eliminate cross-conduction errors among an array of voltage-mode drivers connected in a parallel array. Furthermore, what is needed in the art is a means to ensure very high quality dynamic performance in an array of voltage-mode drivers connected in a parallel array.